1. Field of the Invention
The present invention relates to a gas sensor for measuring oxides such as NO, NO.sub.2, SO.sub.2, CO.sub.2, and H.sub.2 O contained in, for example, atmospheric air and exhaust gas discharged from vehicles or automobiles, and inflammable gases such as CO and CnHm.
2. Description of the Related Art
Those hitherto known as the method for measuring NOx in a measurement gas such as combustion gas include a technique in which the NOx-reducing ability of Rh is utilized in a sensor comprising a Pt electrode and an Rh electrode formed on an oxygen ion-conductive solid electrolyte such as zirconia so that an electromotive force generated between the electrodes is measured.
However, the sensor as described above suffers the following problems. That is, the electromotive force is greatly changed depending on the change in concentration of oxygen contained in the combustion gas as the measurement gas. Moreover, the change in electromotive force is small with respect to the change in concentration of NOx. For this reason, the conventional sensor tends to suffer influence of noise.
Further, in order to bring out the NOx-reducing ability, it is indispensable to use a reducing gas such as CO. For this reason, the amount of produced CO is generally smaller than the amount of produced NOx under a lean fuel combustion condition in which a large amount of NOx is produced. Therefore, the conventional sensor has a drawback in that it is impossible to perform accurate measurement for a combustion gas produced under such a combustion condition.
In order to solve the problems as described above, for example, Japanese Laid-Open Patent Publication No. 8-271476 discloses a NOx sensor comprising pumping electrodes having different NOx-decomposing abilities arranged in a first internal space which communicates with a measurement gas-existing space and in a second internal space which communicates with the first internal space, and a method for measuring the NOx concentration in which the O.sub.2 concentration is adjusted by using a first pumping cell arranged in the first internal space, and NO is decomposed by using a decomposing pumping cell arranged in the second internal space so that the NOx concentration is measured on the basis of a pumping current flowing through the decomposing pump.
Further, Japanese Laid-Open Patent Publication No. 9-113484 discloses a sensor element comprising an auxiliary pumping electrode arranged in a second internal space so that the oxygen concentration in the second internal space is controlled to be constant even when the oxygen concentration is suddenly changed.
In general, the gas sensor is affected by any pulsation or fluctuation of the exhaust gas pressure generated in a measurement gas when the gas sensor is practically used in a vehicle or an automobile. As a result, the oxygen existing in the external space suddenly enters the fist space of the gas sensor.
In order to solve this problem, for example, there have been hitherto adopted an arrangement in which the diffusion rate-determining section provided between the gas-introducing port and the first space is composed of a porous member, and an arrangement in which the diffusion rate-determining section is formed to have a slit-shaped configuration or the diffusion rate-determining section is formed by inserting a porous member into a slit.
However, it has been revealed that the influence of the exhaust gas pressure pulsation cannot be effectively avoided even when the arrangement as described above is adopted, because of the following reason. That is, when the pressure in the external space is positive as compared with the pressure in the first space due to the exhaust gas pressure pulsation, the oxygen in the external space suddenly enters the first space as if it protrudes toward the first space.
The pressure in the external space is negative as compared with the pressure in the first space at the point of time of completion of the protruding supply of oxygen to the first space. However, the oxygen, which has been once supplied to the first space, is not discharged through the diffusion rate-determining section via a route opposite to the supply route. Therefore, in such a case, the oxygen is merely pumped out to the external space by means of the pumping action effected by a main pump. The main pump performs the pumping process so that the oxygen concentration in the first space is a predetermined concentration. However, a certain degree of time is required to allow the oxygen concentration to arrive at the predetermined concentration. During this period of time, a phenomenon occurs, in which the oxygen is suddenly supplied to the first space again due to the exhaust gas pressure pulsation. Consequently, it is impossible to efficiently perform the operation for controlling the oxygen concentration in the first space to be the predetermined concentration by using the main pump.
As a result, it is inevitable that the correlation is deteriorated between the oxygen concentration in the measurement gas and the oxygen pumping amount effected by the main pump in the first space. It is feared that the disturbance of the oxygen concentration in the first space may cause deterioration concerning the control of oxygen concentration in the second space which communicates with the first space, and concerning the measurement accuracy on the detecting electrode which serves as a NOx-detecting section.